Sulfonylureas are oral hypoglycemics widely used in the treatment of Non-Insulin Dependent Diabetes Mellitus (NIDDM). They enter the bloodstream, bind with high affinity to a pancreatic .beta.-cell plasma membrane protein termed the sulfonylurea receptor, and stimulate insulin release. The mechanism of stimulation is thought to be through inhibition of an ATP-sensitive K+ channel (K.sub.ATP), a key protein which sets the .beta.-cell resting membrane potential (Ashcroft, et al. Cell. Signal. 1990, 2, 197-214, all references cited herein are incorporated by reference in their entirety). A reduction in potassium outflow causes depolarization of the plasma membrane, activation of L-type voltage-dependent calcium channels (VDCCs), and increased cytosolic calcium. This triggers insulin release by as yet unknown mechanisms (Rajan, et al. Diabetes Care 1990, 13, 340-363). In NIDDM patients on sulfonylureas, the consequent reduction in blood glucose to more normal levels is thought to be critical in controlling the disease (Gerich, J. E. New Engl. J. Med. 1989, 321, 1231-1245).
The biochemistry of the sulfonylurea receptor (SUR) (Ashcroft et al Biochem. Biophys Acta 1992, 1175, 45-49 and Panten et al. Horm. Metab. Res. 1992, 24, 549-554) is consistent with the electrophysiology of the .beta.-cell K.sub.ATP channel. The endogenous regulators of channel activity include cytosolic nucleotides (ATP and Mg-ADP) and possibly phosphorylation. In the absence of cytosolic nucleotides, sulfonylureas weakly inhibit channel activity (Schwanstecher et al. Br. J. Pharmacol 1992, 107, 87-94). When channels are activated by Mg-ADP, inhibition by ATP is strongly promoted by the presence of sulfonylureas. These results are interpreted as evidence that simultaneous occupancy of two nucleotide binding sites is required for effective channel inhibition by the sulfonylureas. The reported allosteric interactions correlate well with evidence that the brain receptor has two nucleotide binding sites (de Weille, et al. J. Biol. Chem 1992, 267, 4557-4563) physically located on the same polypeptide chain as the sulfonylurea binding site (Bernardi et al. Biochemistry 1992, 31, 6328-6332). One binding site appears to be specific for ATP, and is proposed to be the same site at which micromolar concentrations of ATP inhibit the K.sub.ATP channel. A second site has high affinity for Mg-ADP, with occupancy at this site promoting channel opening. Absolute concentrations of ATP and ADP in the cell are thought to regulate channel activity in a straightforward fashion (Hopkins et al. J. Membrane Biol. 1992, 129, 287-295). High ATP concentrations as a result of high serum glucose levels close the channel, stimulating insulin secretion. Reduced glucose levels increase intracellular ADP concentrations, and thereby increase the open channel probability, and decrease insulin secretion.
Although sulfonylureas, particularly tolbutamide and more potent second generation drugs like glyburide and glipizide, are considered to be relatively specific inhibitors of the K.sub.ATP channel, the exact relationship between the sulfonylurea receptor and the K.sub.ATP channel is not clear (Nichols et al. Am. J. Physiol. 1991, 261, H1675-H1686, Takano et al. Progress in Neurobiology 1993, 41, 21-30, and Edwards et al. Annu. Rev. Pharmacol. Toxicol. 1993, 33, 597-637). In the insulin-secreting CRI-G1 cell line, the addition of glyburide, or tolbutamide to inside-out plasma membrane patches inhibits the K.sub.ATP channel (Khan et al. Proc. R. Soc. Lond. B. 1993, 253, 225-231), intimating direct interactions between sulfonylureas and the channel protein. In another insulin secreting cell line, CRI-D11 cells, however, the loss of sulfonylurea binding sites with the retention of K.sub.ATP activity suggests these two activities may uncouple and reside on separate, transiently bound subunits (Khan et al. Proc. R. Soc. Lond. B. 1993, 253, 225-231). Similarly, in other cell and tissue types, sulfonylurea binding and channel activity may be uncoupled (Ashford et al Br. J. Pharmac. 1990, 101, 531-540). A technique is not currently available to assess whether K.sub.ATP activity resides within the same polypeptide containing the putative nucleotide and sulfonylurea binding sites, or on separate loosely, or tightly bound subunits.
A previous attempt to purify the receptor from hamster insulin-secreting tumor (HIT) cells was limited by the low abundance of the receptor and the presence of a more abundant co-purifying protein. Aguilar-Bryan, L., et al., JBC, 1990, 265, 8218.
The sulfonylurea receptor is the target for drugs used in the treatment of type II diabetes (non-insulin diabetes mellitus). This association has suggested it plays a role in the regulation of insulin secretion by glucose and makes the sulfonylurea receptor a potential diabetes candidate gene.
Persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is an autosomal recessive disorder of glucose homeostasis characterized by unregulated secretion of insulin and profound hypoglycemia. A. Aynsley-Green et al., Arch. Dis. Child. 1981, 56, 496. The pathophysiology of this disease remains obscure, but in vitro studies suggest a defect of glucose-regulated insulin secretion in pancreatic islet .beta.-cells. Aynsley-Green et al., supra., N. Kaiser et al., Diabetologia 1990, 33, 482. The incidence of PHHI has been estimated at 1/50,000 live births in a randomly mating population. G. J. Bruining, Curr. Opin. Pediatr. 1990, 2, 758. However, in a Saudi Arabian population in which 51% of births occurred to parents who were first or second cousins, the incidence has been established as 1/2675 live births. P. M. Mathew et al., Clin. Pediatr. 1988, 27, 148. Recently, the PHHI gene was assigned to chromosome 11p1415.1 by linkage analysis. B. Glaser et al., Nature Genet. 1994, 7, 185 and P. M. Thomas, G. J. Cote, D. M. Hallman, P. M. Mathew, Am. J. Hum. Genet. 1995, 56, 416-421. Candidate genes for this disorder include those involved in the .beta.-cell glucose sensing mechanism and insulin secretion. Localization of PHHI to chromosome 11p excluded previously mapped genes involved in .beta.-cell function. Considered as a candidate was the newly cloned high-affinity SUR gene, a member of the ATP-binding cassette superfamily, and a putative subunit of the modulator of insulin secretion, the .beta.-cell ATP-sensitive potassium channel (K.sub.ATP). S. J. Ashcroft and F. M. Ashcroft, Biochimica et Biophysica Acta, 1992, 1175, 45; U. Panten, M. Schwanstecher, and C. Schwanstecher, Horm. Metab. Res. 1992, 24, 549. The methods of the present invention map the sulfonylurea receptor to the same chromosomal location as PHHI and provide evidence that mutations in the sulfonylurea receptor are the cause of PHHI.
Accordingly, there remains a need to identify sulfonylurea receptor and sequences encoding sulfonylurea receptor which will provide:
1. a correlation between sulfonylurea receptor and one or more forms of diabetes, PA1 2. a sequence to purify human sulfonylurea receptors, PA1 3. an isolated sulfonylurea receptor, prepared by recombinant methods, PA1 4. polyclonal and monoclonal antibodies and methods of preparing the same against sulfonylurea receptor, PA1 5. information as to whether this receptor-ion channel family involves multi-subunits within each channel for channel activity, PA1 6. gene therapy such that sequences which encode mutant sulfonylurea receptors are replaced by wild type sulfonylurea receptor sequences, PA1 7. a method of screening to identify drugs which react with and bind to the sulfonylurea receptor, PA1 8. non-human transgenic animals to study diabetes and PHHI, and the physiologic effects of varying levels of sulfonylurea receptor, by using an inducible promoter to regulate the expression of the sulfonylurea receptor, for example, and PA1 9. probes, including PCR probes, for diagnosing conditions associated with the expression of a specific sulfonylurea receptor allele.
The present invention reveals that the sequence encoding the mammalian sulfonylurea receptor maps to the sequence encoding persistent hyperinsulinemic hypoglycemia of infancy.